This invention relates to remotely controlled electro-mechanical RF impedance tuners used in load and source pull testing of high power RF transistors and amplifiers.
Modern design of high power RF amplifiers used in various communication systems, requires accurate knowledge of the active device's (microwave transistor's) characteristics. In such circuits, it is insufficient for the transistors, which operate in their highly non-linear regime, close to power saturation, to be described using non-linear numeric models only.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull” (FIG. 1). Load pull is a measurement technique employing microwave impedance tuners 2, 4 and other microwave test equipment, such as signal sources 1, test fixtures and DUT 3 and power meters 5, the whole controlled by a computer 6; said computer controlling and communicating with said tuners 2, 3 and other equipment 1, 3 and 5 using digital cables 7, 8 and 9. The tuners are used in order to manipulate the microwave impedance conditions under which the Device Under Test (DUT, or transistor) is tested (see ref. 1); tuners allow determining the optimum impedance conditions for designing amplifiers and other microwave components for specific performance targets, such as gain, efficiency, inter-modulation etc.; this document refers hence to “tuners” as being “impedance tuners”, in order to distinct from “tuned receivers (radios)”, commonly referred to as “tuners” because of the included tuning circuits (see ref. 2).
Impedance tuners comprise, in general, a slotted transmission airline (slabline) 23, 24 and adjustable reflection probes 22, FIG. 2; the probe 22 is attached to a precision vertical axis 21 which is mounted inside a mobile carriage 28; the axis 21 can move the probe 22 vertically 216 towards the center conductor 23 and the carriage 28 can move the probe 22 horizontally 217 parallel to the center conductor 23 of the slabline 24. The vertical movement 216 changes the amplitude of the reflection factor seen at the tuner test port 25 whereas the horizontal movement 217 changes the phase. This way the whole impedance plan (Smith chart) is covered allowing a quasi-infinity number of impedances from Zmin to Zmax to be synthesized at any given frequency within the “tuning range” of the tuner. Typical values of state of the art tuners are |Zmin|≈2Ω and |Zmax|≈1250Ω; this corresponds to Voltage Standing Wave Ratio (VSWR) of 25:1 (FIG. 6). The relation between reflection factor and impedance is given by GAMMA=|GAMMA|*exp(jΦ), (Z−Zo)/(Z+Zo) {eq.1}, wherein Z is the complex impedance Z=R+jX and Zo is the characteristic impedance. A typical value used for Zo is Zo=50Ω (see ref. 3). The equivalent to GAMMA is the Voltage Standing Wave Ratio: VSWR=(1+|GAMMA|)/(1−|GAMMA|) {eq.2}.
Metallic probes 22, 30 or “slugs” are typically made in a cubical form 41 with a concave bottom 35 which allows to capture, when approaching the center conductor 32 (see FIG. 11 in ref. 4), the electric field, which is concentrated in the area 36 between the center conductor 32 and the ground planes of the slotted airline (slabline) 31, FIG. 3. This “field capturing” allows creating controllable high capacitance and reflection factors. The critical part is the required high proximity of the probe to the center conductor and the high accuracy of the vertical probe movement (FIG. 6), whereby changes in the vertical probe position 62 of a few micrometers affects the reflection factor (and the VSWR) by a large amount.
When the center conductor 53 of the slabline heats up, the solution disclosed here prevents it from bending 57 and eventually “buckling”. This is obtainable if the center conductor is allowed to expand axially 105, 106, which it cannot do, if not modified, as shown in FIG. 5, because its expansion is limited by the connectors 518, 519. The prior art configuration of FIG. 9, whereby a protrusion 91 machined out of the main segment 94 of the center conductor slides into a hole 92 drilled into the fixed portion 93, does allow the center conductor to expand, but the required gap 96 must be at least twice as large as in the case of the present invention which is depicted in FIG. 10, whereby the gap is split in two, approximately equal, parts 104 and 109. Also, in the configuration of FIG. 9 the unmoving segment 93 of the center conductor is a short piece of center conductor, anchored permanently on the connector plate 95, and much shorter than the moving part 94 of the center conductor. This means not only that the thermal expansion of the free part 94 is larger (typically at least twice as long as in the case of FIG. 10, since its length is larger, thermal expansion of a rod is proportional to its length), but also that, because of its larger length, the slenderness factor “Length/Diameter” is larger. In the case of FIG. 10, if the joining tube 102 is placed in the middle of the center conductor, i.e. each free-standing segment is one half as long as the tuner (Length/2), then the slenderness factor will also be half, and, following Euler's formula (see ref. 5), the required force for the center conductor to bend or “buckle” will be four times larger. Therefore the risk of “buckling” under axial force will be four times smaller.
When microwave power is injected into the tuner, some of it is absorbed by the center conductor 53 of the slabline. This leads to a rise of its temperature and associated thermal expansion mostly along its axis. Since the center conductor 53 has only limited range for expansion, because it butts at the connectors 518 and 519, this leads to bending and eventually “buckling” 57 in FIG. 5A; “Buckling” of the center conductor happens in different ways, depending on its pre-forming, which cannot be “perfectly” straight, its “slenderness” factor Length/Diameter and the anchoring method at the connectors. Whereas in an ideal, low power and room temperature, situation FIG. 5B1, the center conductor is positioned exactly in the center of the slabline channel, when heated and it bends (deflects) it may either deflect sideways 515 or downwards 514 or both; of course, it may also deflect upwards (not shown), in which case we may have a premature electrical short circuit. In either case the effect is “at best” loss of accuracy or “at worst” an electrical short and damage of the tuner and/or the DUT.
In case a short happens with either temporary or permanent damage of the tuner or the DUT, at least the operator will be alerted and can take measures to correct the situation. But if it does not come to a short (case of FIG. 5B3), then the result will “only” be false measurement. This is because, as shown in FIG. 6, a relative movement between tuner probe and center conductor will change the calibrated VSWR; in other words, the data retrieved from the measurement instruments (FIG. 1) will be recorded at the wrong VSWR values. There will be no warning, just wrong data; it comes even worse: would the operator have doubts about the tuner accuracy, and would he disconnect the tuner from the test setup in order to re-calibrate or verify the calibration on a vector network analyzer (VNA), he, probably, will find that the tuner is accurate. This is because, during the dismantling of the setup the center conductor will cool down and recover its initial (calibrated) position (transition from states FIG. 5B2 or FIG. 5B3 back to original state FIG. 5B1); this can be a substantial systemic problem for high power testing using such tuners.
This invention discloses solutions allowing avoidance of such catastrophic and systemic test problems.